Currently in the semiconductor industry there is great interest in monitoring the presence of charge in insulating layers deposited onto semiconductor surfaces. Such charges may be indicative of contamination or defects in these layers which may affect the performance of semiconductor devices.
Current techniques for measuring charge in insulating layers include the following.
A first technique is capacitance-voltage profiling in which the capacitance of an electrode in intimate contact with a sample is measured as a function of an applied bias voltage and, possibly, frequency. References in this regard are S. M. Sze, "Physics of Semiconductor Devices", New York: John Wiley and Sons, 1969, and also A. S. Grove "Physics and Technology of Semiconductor Devices", New York: John Wiley and Sons, 1967. In a variation of this method a small amount of liquid mercury may be placed in contact with the sample through the use of a small capillary.
A disadvantage of this technique is that it is necessary to have an electrode in contact with the sample.
A second known approach employs a surface photo-voltage technique. In this technique a voltage is applied to the surface of the sample by means of an electrode, and the sample is illuminated with a low-intensity light source, such as a light-emitting diode, whose intensity is modulated at a low frequency, for example 10 kHz.
A disadvantage of this technique is that either a contacting electrode is required or, alternatively, an electrode which can deposit charge onto the surface of the sample.
A further approach is known as Deep Level Transient Spectroscopy (DLTS). In this technique the temperature is slowly swept and the charges are progressively released from their trapping sites. The resulting change in capacitance is measured to infer the density of charge trapping centers.
However, this technique also requires that an electrical contact be made to the sample.
Furthermore, none of techniques are well-suited to the study of very small areas of a sample because the sensitivity decreases with decrease in the area that is probed.
In the semiconductor industry certain materials such as silicon, germanium, and gallium arsenide are frequently doped with impurities so as to change their electrical or mechanical properties. These impurities may be introduced by means of ion implantation or by means of in-diffusion from a solid, liquid or gas source. Associated with the introduction of such impurities is an amount of crystalline damage whose characteristics depend on the method by which they are introduced. A variety of ions are commonly used for this purpose including B, P, Ga, Ge, F, Si, B11, BF2, Sb, In, As and hydrogen. In the case of implantation, these ions are accelerated to an energy which may be as low as a few keV or as high as several hundred keV, and are then directed at the surface of the material. After entering the material an ion loses energy by collisions with the atoms of the material. These collisions result in damage to the material, such as displacements of atoms from their normal crystalline positions. For sufficiently high ion doses parts of the material may become amorphous rather than crystalline. The material is thus modified as a result of the damage that occurs (also referred to as the generation of defect sites) and as a result of the introduction of the ions themselves, even if no damage occurs. For in-diffused species, crystal damage in the sample, such as a substrate, may occur as the diffusing atoms displace sample atoms from their lattice sites. The extent of the damage depends on the size of the sample and the diffusing atoms, the nature of the diffusion source (solid, liquid, gas), the concentration of diffusion species in the source, and the details of the thermal process used to drive them into the substrate. It is also possible for there to be no crystal damage (e.g. if the diffusing atoms are small compared to the lattice constant of the sample). In such cases, diffusing atoms may occupy interstitial sites in the sample, and so may alter the local electronic and optical characteristics of the sample.
The material modification generally occurs in a surface layer or region the depth of which can vary from less than 100 Angstroms for low energy ions to several microns (e.g. when high energy ions are used). The dosage, i.e. the number of ions introduced per unit area of the surface of the material, can be varied over a wide range for implanted species by controlling the ion beam current and the time for which the ion beam is directed at the material. For the in-diffusion case the dosage can be controlled varying the thermal cycle or the source concentration. Currently in the semiconductor industry, implant doses as low as 10.sup.10 ions per cm.sup.2 and as high as 10.sup.18 ions per cm.sup.2 are used for different purposes. Both the material damage and the introduction of the ions results in a change in the electrical properties of the material in the vicinity of the surface where the ions are introduced. Some of the damage to the crystalline structure can be removed by thermally annealing the material.
In the fabrication of semiconductor chips, ion implantation or in-diffusion may be used at a number of stages of the process. Typically, an implant is restricted to predetermined areas, i.e. the implant is patterned. Similarly, in-diffused species may be added in a pattern by masking regions with an impenetrable, heat resistant layer such as SiO.sub.2 or nitride. It is important to be able to monitor the dosage and to confirm that the correct regions have been implanted or doped by in-diffusion. Since these regions may be very small, it is important for a measurement technique to have very high spatial resolution. Also, and to avoid unintentionally contaminating the sample during the measurement, it is desirable that a non-contact measurement method be used.
A number of different techniques have been used or proposed for the evaluation of ion-implanted materials, including Rutherford back-scattering, Raman spectroscopy, and sheet resistance measurements. Some of these techniques have also been used to characterize samples to which foreign atoms have been introduced by in-diffusion.
Yet another technique which has been used to characterize ion implants employs a 100% intensity modulated laser beam with modulation frequency .omega. that is directed at a semiconductor surface, as described by Opsal et al., Method and Apparatus For Evaluating Surface and Subsurface Features in a Semiconductor", U.S. Pat. No. 4,854,710. The light that is absorbed in the sample generates an electron-hole plasma, and also a heavily damped thermal wave close to the surface of the sample. Both the plasma and the thermal wave oscillate at frequency .omega.. These forced plasma and thermal oscillations give rise to small oscillations in the optical reflectivity of the sample which can be measured by means of a probe laser directed onto the same spot as the modulated laser. The amplitude and phase of the small oscillatory component at frequency .omega. arising in the intensity of the reflected probe beam depend strongly on .omega., and also can be affected by the presence of ion implants and related damage in the semiconductor. Thus a measurement of this oscillatory component can be used as a defect or ion implant monitor.
Reference in this regard can also be had to J. Opsal, "Method and Apparatus for Evaluating Ion Implant Levels in Semiconductors", U.S. Pat. No. 5,074,669. In this technique, both the unmodulated component of the reflected probe beam, and the component modulated at frequency .omega., are measured and analyzed. In all of the above described techniques the modulation frequency of the pump beam is typically below 10 MHz.
Photo-acoustic displacement measurements (PAD) have also been shown to be sensitive to ion-implant dosage, as described by S. Sumie et al., Jap. J. Appl. Phys. 35, 3575 (1992), and S. Sumie, et al., J. Appl. Phys. 76, 5681 (1994). In these experiments the acoustic displacement is periodic at a frequency of 87 kHz. The measurement is designed so that changes in optical reflectivity due to the electrons and holes excited in the material are not detected.
The optical methods mentioned above generally use periodically-modulated continuous wave pump beams to excite the material. The frequency of the modulation is typically in the range below 10 MHz. However, this range of modulation frequencies can adversely impact the sensitivity of the measurement system and an ability to "profile" the impurities or damage distribution, and may also cause the system to be sensitive to surface effects.
The thermal and electrical properties of materials have also been studied using optical pulse techniques. Short light pulses (duration 100 psec or less) have been used to heat a metal film on a semiconductor dielectric substrate. A time-delayed probe pulse (duration also 100 psec or less) is used to measure the change in the optical reflectivity of the metal film, and from this change the rate at which the film cools by thermal conduction into the substrate can be determined. Reference in this regard can be had to Young et al., Heat Flow in Glasses on a Picosecond Timescale in Phonon Scattering in Condensed Matter V, edited by A. C. Anderson and J. P. Wolfe, (Springer, Berlin, 1986), p. 49; to Stoner et al., Measurements of the Kapitza Conductance between Diamond and Several Metals Phys. Rev. Lett. 68, 1563 (1992); and to Stoner and Maris, Kapitza Conductance and Heat Flow Between Solids at Temperatures from 50 to 300 K, Phys. Rev. B48, 16373 (1993).
Short light pulses have been used to excite electrons and holes in semiconductors, and the change in optical reflectivity that occurs as a result of the excited carriers has been measured with a short probe light pulse. In this regard reference can be made to Auston et al., Picosecond Ellipsometry of Transient Electron-Hole Plasmas in Germanium, Phys. Rev. Lett. 32, 1120 (1974); to Auston et al., Picosecond Spectroscopy of Semiconductors, Solid State Electronics 21, 147 (1978); and to Elci et al., Physics of Ultrafast Phenomena in Solid State Plasmas, Solid State Electronics 21, 151 (978)). This work has generally been directed towards achieving an understanding of how the electrons and holes relax and diffuse, rather than as a means for sample characterization.
In a paper entitled "Carrier Lifetime Versus Ion-Implantation Dose in Silicon on Sapphire", F. E. Doany et al., Appl. Phys. Lett. 50(8), Feb. 23, 1987 (pp. 460-462), a report is made of studies conducted on a silicon film of thickness 0.5 micron on a sapphire substrate. The authors employed 70 femtosecond pulses that were generated at a 100 MHz rate, the pump pulses are said to be chopped at a 1 kHz rate, and the probe pulses were obtained from the pump pulses. A change in reflectivity over time was obtained from a photodetector. In this experiment the excited carriers could not enter the substrate because of the large band gap of the sapphire, and hence were confined to the silicon film. Consequently, the electrons and holes were distributed approximately uniformly throughout the thickness of the silicon film, and this assumption was made in the analysis of the data by these authors. It was demonstrated that the lifetime of the excited free carriers was influenced by the implantation dose of O.sup.+ ions, and that there is lack of carrier lifetime dependence above an O.sup.+ implant dose of 3.times.10.sup.14 cm.sup.-2. It is important to note that in this approach the generated heat cannot readily dissipate and the temperature of the sample can become high.
In a nondestructive ultrasonic technique described in U.S. Pat. No. 4,710,030 (Tauc et al.), a very high frequency sound pulse is generated and detected by means of an ultrafast laser pulse. The sound pulse is used to probe an interface. The ultrasonic frequencies used in this technique typically are less than 1 THz, and the corresponding sonic wavelengths in typical materials are greater than several hundred Angstroms. It is equivalent to refer to the high frequency ultrasonic pulses generated in this technique as coherent longitudinal acoustic phonons.
In more detail, Tauc et al. teach a system wherein a transient optical response arises from mechanical waves (stress pulses) which are generated by a pump pulse and which propagate in the sample. Tauc et al. describe the use of pump and probe beams having durations of 0.01 to 100 psec. These beams may impinge at the same location on a sample's surface, or the point of impingement of the probe beam may be shifted relative to the point of impingement of the pump beam. In one embodiment the film being measured can be translated in relation to the pump and probe beams. The probe beam may be transmitted or reflected by the sample. In a method taught by Tauc et al. the pump pulse has at least one wavelength for non-destructively generating a stress pulse in the sample. The probe pulse is guided to the sample to intercept the stress pulse, and the method further detects a change in optical constants induced by the stress pulse by measuring an intensity of the probe beam after it intercepts the stress pulse.
In one embodiment a distance between a mirror and a corner cube is varied to vary the delay between the impingement of the pump beam and the probe beam on the sample. In a further embodiment an opto-acoustically inactive film is studied by using an overlying film comprised of an opto-acoustically active medium, such as arsenic telluride. In another embodiment the quality of the bonding between a film and the substrate can be determined from a measurement of the reflection coefficient of the stress pulse at the boundary, and comparing the measured value to a theoretical value.
The methods and apparatus of Tauc et al. are not limited to simple films, but can be extended to obtaining information about layer thicknesses and interfaces in superlattices, multilayer thin-film structures, and other inhomogeneous films. Tauc et al. also provide for scanning the pump and probe beams over an area of the sample, as small as 1 micron by 1 micron, and plotting the change in intensity of the reflected or transmitted probe beam.